$(3$ points$)$ Question $2$: What is a state$-$space tree? a$.)$ A representation of a problem with nodes as partial solutions and leaves as $($potential$)$ solutions. b$.)$ A representation of a problem with leaves as partial solutions and nodes as $($potential$)$ solutions. c$.)$ A representation of a problem such that solutions are always found in the left$-$most branch. d$.)$ A representation of a problem such that solutions are always found in the right$-$most branch. e$.)$ A representation of a problem such that solutions are made to bubble up to the root.